The present invention relates to a controller for an electric four-wheel-drive vehicle that runs while its wheels are driven by engine-generated driving force and its other wheels are driven by motor-generated driving force.
In recent years, an increasing number of automobiles are motor-driven. They are electric automobiles, hybrid automobiles, and other environment-responsive automobiles and electric four-wheel-drive vehicles. These vehicles are mainly characterized by the fact that they carry a battery or engine-driven generator and use the power generated by the battery or generator to drive their wheels with a motor. When front wheels of a vehicle are motor-driven, the vehicle is operated as a front-wheel-drive vehicle so that the motor assists an engine shaft with torque. When, on the other hand, rear wheels of the vehicle are motor-driven, the vehicle is operated as an electric four-wheel-drive vehicle because the front wheels are engine-driven. The electric four-wheel-drive vehicle is an electric automobile that excels in low-micron road run performance and turning performance because its rear wheels are motor-driven. Since it has a dedicated alternator and does not incorporate a battery, it is a low-cost electrical drive system that imposes favorable assembly conditions.
In a system employed by the above-mentioned electric automobiles, hybrid automobiles, and electric four-wheel-drive vehicles, electrical power always flows between the generator and drive motor/inverter. In a system without a battery, for example, electrical power is steadily supplied without causing an electrical power imbalance if the power generated by the generator is equal to the power consumption of (the input power for) the motor/inverter. However, if the power generated by the generator is larger than the power consumption of (the input power for) the motor/inverter, excessive power is generated to charge a smoothing capacitor and raise the voltage of a DC bus. The smoothing capacitor is positioned between the generator and inverter to smooth the input voltage for the inverter. On the contrary, if the power generated by the generator is smaller than the power consumption of (the input power for) the motor/inverter, power insufficiency results. Consequently, the smoothing capacitor may be discharged to lower the voltage of the DC bus (the voltage of an electrical power bus between the generator and inverter), thereby causing torque insufficiency.
A method for furnishing a solution to a situation where the power generated by the generator is larger than the power consumption of (the input power for) the motor/inverter is disclosed, for instance, by JP-A-2005-143285. When excessive power is generated by the generator, the disclosed method consumes the excessive power in a hybrid transmission having at least two motor generators by increasing the torque of a third motor, which is provided in a vehicle drive system. The use of this method makes it possible to inhibit the generator from generating excessive power.